Polyethylene is the most widely used commercial polymer and can be prepared by different processes. Polymerization in the presence of free-radical initiators at elevated pressures was the method first discovered to obtain polyethylene and continues to be a valued process, with high commercial relevance for the preparation of low density polyethylene (LDPE).
A common set-up of a production line for preparing low density polyethylene comprises a polymerization reactor which can be an autoclave or a tubular reactor or a combination of such reactors and equipment. For pressurizing the reaction components, a set of two compressors, a primary compressor and a secondary compressor, may be used. At the end of the polymerization sequence, a production line for high-pressure polymerization may further include apparatuses like extruders and granulators for pelletizing the resulting polymer. Furthermore, such a production line may also comprise means for feeding monomers and comonomers, free-radical initiators, modifiers or other substances at one or more positions to the polymerization reaction.
A characteristic of the radically initiated polymerization of ethylenically unsaturated monomers under high pressure is that the conversion of the monomers is often incomplete. For each pass of the reactor or the reactor combination, only about 10% to 50% of the dosed monomers are converted in the case of a polymerization in a tubular reactor, and from 8% to 30% of the dosed monomers are converted in the case of a polymerization in an autoclave reactor. The resulting reaction mixture often leaves the reactor through a pressure control valve and may then be separated into polymeric and gaseous components with the unreacted monomers being recycled. To avoid unnecessary decompression and compression steps, the separation into polymeric and gaseous components can be carried out in at least two stages. The monomer-polymer mixture leaving the reactor is usually transferred to a first separating vessel, frequently called high-pressure product separator, in which the separation in polymeric and gaseous components is carried out at a pressure that allows for recycling the ethylene and comonomers separated from the monomer-polymer mixture to the reaction mixture at a position between the primary compressor and the secondary compressor. At the conditions of operating the first separation vessel, the polymeric components within the separating vessel are in liquid state. The liquid phase obtained in the first separating vessel is transferred to a second separation vessel, frequently called low-pressure product separator, in which a further separation in polymeric and gaseous components takes place at lower pressure. The ethylene and comonomers separated from the mixture in the second separation vessel are fed to the primary compressor where they are compressed to the pressure of the fresh ethylene feed, combined with the fresh ethylene feed and the joined streams are further pressurized to the pressure of the high-pressure gas recycle stream.
The polymerization process in a LDPE reactor is carried out at high pressures which can reach 350 MPa. Such high pressure may require special technology for the process to be handled in a safe and reliable manner. Technical issues in handling ethylene at high pressures are, for example, described in Chem. Ing. Tech. 67 (1995), pages 862 to 864. It is stated that ethylene decomposes rapidly in an explosive manner under certain temperature and pressure conditions to give soot, methane and hydrogen. This undesired reaction occurs repeatedly in the high-pressure polymerization of ethylene. The drastic increase in pressure and temperature associated therewith represents a considerable potential risk for the operational safety of the production plants.
A possible solution for preventing a drastic increase in pressure and temperature of this type involves installing rupture discs or emergency pressure-relief valves. WO 02/01308 A2, for example, discloses a specific hydraulically controlled pressure relief valve which allows a particularly fast opening of the pressure relief valve in case of sudden changes in pressure or temperature. It is technically possible to handle such thermal runaways or explosive decompositions of ethylene within the polymerization reactor, however these situations are highly undesirable since thermal runaways or explosive decompositions of ethylene within the polymerization reactor lead to a shut-down of the polymerization plant with frequent emission of ethylene into the environment and loss of production.
Another threat to the operational safety of high-pressure polymerization plants is the occurrence of leaks. Due to the high pressure difference between the interior of the polymerization reactor and the surrounding, even small fissures in a wall of high-pressure equipment may lead to an exit of a considerably high amount of the reactor content resulting in locally high concentrations of combustible hydrocarbons in a short time period. On the other hand, in case of larger leaks, the available time for reacting is extremely short.
Processes for polymerizing or copolymerizing ethylenically unsaturated monomers at pressures in the range of from 110 MPa to 500 MPa places specific demands on a reliable detection of combustible or explosive gases, which may leak from the polymerization equipment. Depending on the size and the position of the leak, the leakage rate may be extremely high or relatively low, with a risk of an accumulation of the leaked material. The leaked material may have a temperature in the range from 100° C. to 350° C. but can also be cold and may therefore sink to the ground and accumulate there. The concentration of leaked gas in a certain volume element in the vicinity of the polymerization plant can vary from substantially pure hydrocarbon to a very low concentration of combustible gas in air. Furthermore, the leakage can not only take place towards the atmosphere but can also occur at a section of the equipment which is covered by a cooling or heating jacket. Moreover, as such processes are not carried out in totally closed housings, also weather phenomena such as wind and rain may have an influence on the detection of leaked gases.
Another difficulty with respect to processes for preparing ethylene polymers at high pressure is that the reaction mixture is a supercritical composition comprising monomer and polymer. After a leakage of such a reaction mixture into the atmosphere, small polymer particles are formed which are subject to electrostatic charging. Consequently, there is an enhanced probability for an ignition after an explosive gas cloud has developed after an escape of reaction mixture.
WO 2008/148758 A1 discloses a method of operating a high-pressure ethylene polymerization unit comprising a tubular reactor equipped with a cooling jacket, in which method the leakage of reaction mixture into the cooling jacket is controlled by monitoring the electrical conductivity of the aqueous cooling medium. Such a method however requires that at least one of the chemical substances in the reaction mixture changes the electrical conductivity of the aqueous cooling medium and furthermore can only detect leakage at positions of the polymerization equipment which are covered by a cooling jacket.
The polymerization of ethylenically unsaturated monomers at pressures in the range of from 110 MPa to 500 MPa can be carried out in tubular polymerization reactors because these reactors have a high surface for removing the liberated heat of polymerization. Tubular reactors may have a length in the range from about 0.5 km to 5 km and are composed of thick-walled tubes of a length from 5 m to 25 m. The individual tubes of the tubular reactor are flanged together, either directly or via bends. Also, pre-heaters or pre-coolers or post reactor coolers in production lines for preparing low density polyethylenes may be composed of thick-walled tubes of a length from 5 m to 25 m, which are flanged together. Among the parts of a low density polyethylene production line which have a relatively high likelihood of a leakage are these flange connections because these sometimes need to be loosened and retightened for maintenance reasons. However, the leaking rate for a leakage at a flange connection may be relatively small and makes it difficult to quickly recognize the occurrence of such a leak.
Accordingly there is a need to overcome these and other disadvantages and provide a process which allows for a fast recognition of a leakage in a flange of a production line for polymerizing ethylenically unsaturated monomers at pressures in the range of from 110 MPa to 500 MPa.